The present invention relates generally to electrical fused and particularly to fuses of the current limiting type in which the sand filler is bound into a rigid matrix.
A current limiting fuse typically includes a tubular insulating casing closed at both ends by metal terminal caps. Connected to the terminal caps and passing through the interior of the casing is at least one fusible wire or ribbon element, usually of silver. The fusible elements may in some cases be wound around a supporting core which extends axially in the casing between the end caps. The space around the elements and core is filled with a tightly packed arc-quenching filler, usually quartz sand.
In certain fuses known as the "bound filler" type, the sand particles, are bonded together by, for instance, colloidal silica particles so that the filler is in the form of a rigid matrix. The rigid nature of the filler matrix makes the fuse more rugged and, among other things, improves the low current clearing characteristics of the fuse by increasing the burn back rate of the fusible element and by increasing the thermal conductivity of the filler sufficiently to permit a reduction of the physical size of a fuse for a given current rating. Such a fuse is described in detail in the U.S. Pat. No. 3,838,375 issued 24 Sept. 1974 to Frind et al. and assigned to the same assignee as are the rights to the present invention.
It is important that when the fuse clears at low current that the initial fusing of the element takes place at the central portion of the fuse, so that neither end of the element will burn all the way back to its terminal cap. Otherwise, arcing at the terminal cap can result in a failure mode for the fuse.
For fuses with ribbon elements having regularly spaced perforations therein to provide cross-sectional "necks" which melt first, it is common practice to place an overlay of a low fusing point solder near the center of the element to assure initial melting there. Then, as the element reaches an elevated temperature near the fusing temperature of the solder overlay, the solder alloys with the silver. The alloying of the silver increases the resistivity and hence heating of the element there. The overlay is generally placed adjacent a centrally located one of the perforations.
One problem with the bound sand fuses has been that the increased conductivity of heat through the rigid matrix as compared to an unbound filler permits a higher current to flow through the fuse without raising the temperature of the ribbon and overlay sufficiently to initiate the alloying process at the desired current and time. Decreasing the size of the ribbon element to compensate for the higher current carrying capability of the ribbon in the rigid matrix would, however, change the currenttime characteristic of the fuse by reducing the time lag for all but the relatively high currents. This would offset one of the specific purposes of the solder overlay, which is to change the shape of the time-current characteristic curve by increasing the lag time for lower currents while leaving the short lag for high currents unaffected.